<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <html> <head> <meta http-equiv="content-type" content="text/html; charset=UTF-8"> <title>Syntax</title> </head> <body><div class="manualnavbar" style="text-align: center;"> <div class="prev" style="text-align: left; float: left;"><a href="language.constants.html">Constants</a></div> <div class="next" style="text-align: right; float: right;"><a href="language.constants.predefined.html">Magic constants</a></div> <div class="up"><a href="language.constants.html">Constants</a></div> <div class="home"><a href="index.html">PHP Manual</a></div> </div><hr /><div id="language.constants.syntax" class="sect1"> <h2 class="title">Syntax</h2> <p class="simpara"> You can define a constant by using the <span class="function"><a href="function.define.html" class="function">define()</a></span>-function or by using the <em>const</em> keyword outside a class definition as of PHP 5.3.0. Once a constant is defined, it can never be changed or undefined. </p> <p class="simpara"> Only scalar data (<span class="type"><a href="language.types.boolean.html" class="type boolean">boolean</a></span>, <span class="type"><a href="language.types.integer.html" class="type integer">integer</a></span>, <span class="type"><a href="language.types.float.html" class="type float">float</a></span> and <span class="type"><a href="language.types.string.html" class="type string">string</a></span>) can be contained in constants. It is possible to define constants as a <span class="type"><a href="language.types.resource.html" class="type resource">resource</a></span>, but it should be avoided, as it can cause unexpected results. </p> <p class="simpara"> You can get the value of a constant by simply specifying its name. Unlike with variables, you should <em class="emphasis">not</em> prepend a constant with a <em>$</em>. You can also use the function <span class="function"><a href="function.constant.html" class="function">constant()</a></span> to read a constant's value if you wish to obtain the constant's name dynamically. Use <span class="function"><a href="function.get-defined-constants.html" class="function">get_defined_constants()</a></span> to get a list of all defined constants. </p> <blockquote class="note"><p><strong class="note">Note</strong>: <span class="simpara"> Constants and (global) variables are in a different namespace. This implies that for example <strong><code>TRUE</code></strong> and <var class="varname"><var class="varname">$TRUE</var></var> are generally different. </span> </p></blockquote> <p class="simpara"> If you use an undefined constant, PHP assumes that you mean the name of the constant itself, just as if you called it as a <span class="type"><a href="language.types.string.html" class="type string">string</a></span> (CONSTANT vs "CONSTANT"). An error of level <a href="ref.errorfunc.html" class="link">E_NOTICE</a> will be issued when this happens. See also the manual entry on why <a href="language.types.array.html#language.types.array.foo-bar" class="link">$foo[bar]</a> is wrong (unless you first <span class="function"><a href="function.define.html" class="function">define()</a></span> <em>bar</em> as a constant). If you simply want to check if a constant is set, use the <span class="function"><a href="function.defined.html" class="function">defined()</a></span> function. </p> <p class="para"> These are the differences between constants and variables: <ul class="itemizedlist"> <li class="listitem"> <span class="simpara"> Constants do not have a dollar sign (<em>$</em>) before them; </span> </li> <li class="listitem"> <span class="simpara"> Constants may only be defined using the <span class="function"><a href="function.define.html" class="function">define()</a></span> function, not by simple assignment; </span> </li> <li class="listitem"> <span class="simpara"> Constants may be defined and accessed anywhere without regard to variable scoping rules; </span> </li> <li class="listitem"> <span class="simpara"> Constants may not be redefined or undefined once they have been set; and </span> </li> <li class="listitem"> <span class="simpara"> Constants may only evaluate to scalar values. </span> </li> </ul> </p> <p class="para"> <div class="example" id="example-113"> <p><strong>Example #1 Defining Constants</strong></p> <div class="example-contents"> <div class="phpcode"><code><span style="color: #000000"> <span style="color: #0000BB"><?php<br />define</span><span style="color: #007700">(</span><span style="color: #DD0000">"CONSTANT"</span><span style="color: #007700">, </span><span style="color: #DD0000">"Hello world."</span><span style="color: #007700">);<br />echo </span><span style="color: #0000BB">CONSTANT</span><span style="color: #007700">; </span><span style="color: #FF8000">// outputs "Hello world."<br /></span><span style="color: #007700">echo </span><span style="color: #0000BB">Constant</span><span style="color: #007700">; </span><span style="color: #FF8000">// outputs "Constant" and issues a notice.<br /></span><span style="color: #0000BB">?></span> </span> </code></div> </div> </div> </p> <p class="para"> <div class="example" id="example-114"> <p><strong>Example #2 Defining Constants using the <em>const</em> keyword</strong></p> <div class="example-contents"> <div class="phpcode"><code><span style="color: #000000"> <span style="color: #0000BB"><?php<br /></span><span style="color: #FF8000">// Works as of PHP 5.3.0<br /></span><span style="color: #007700">const </span><span style="color: #0000BB">CONSTANT </span><span style="color: #007700">= </span><span style="color: #DD0000">'Hello World'</span><span style="color: #007700">;<br /><br />echo </span><span style="color: #0000BB">CONSTANT</span><span style="color: #007700">;<br /></span><span style="color: #0000BB">?></span> </span> </code></div> </div> </div> </p> <blockquote class="note"><p><strong class="note">Note</strong>: <p class="para"> As opposed to defining constants using <span class="function"><a href="function.define.html" class="function">define()</a></span>, constants defined using the <em>const</em> keyword must be declared at the top-level scope because they are defined at compile-time. This means that they cannot be declared inside functions, loops or <em>if</em> statements. </p> </p></blockquote> <p class="simpara"> See also <a href="language.oop5.constants.html" class="link">Class Constants</a>. </p> </div><hr /><div class="manualnavbar" style="text-align: center;"> <div class="prev" style="text-align: left; float: left;"><a href="language.constants.html">Constants</a></div> <div class="next" style="text-align: right; float: right;"><a href="language.constants.predefined.html">Magic constants</a></div> <div class="up"><a href="language.constants.html">Constants</a></div> <div class="home"><a href="index.html">PHP Manual</a></div> </div></body></html>